In order to increase the capacity of a power conversion system such as a motor drive apparatus, there is a need for increasing the capacity of a power converting apparatus. One known technique for increasing the capacity of the power converting apparatus is arranged to operate a plurality of power converters in parallel connection and to provide, to a motor, a sum of output powers of the power converters.
In one technique for the parallel operation of power converters, power converters are connected, through one or more reactors or interphase reactors, with a motor. In the case of a common power source common to the parallel connected power converters, there arises a voltage difference due to nonuniformity in switching characteristic among the power converters since each power converter is electrically connected. This voltage difference causes an unwanted circulating current among the power converters, and this circulating current is called “cross current”.
Hereinafter, in this description, a system assumed is a PWM power converter parallel running apparatus which has parallel inverters and a cross current compensating function, and which is arranged so that voltage saturation may occur.
FIG. 5 is a block diagram showing one example of a general PWM power converter parallel running or driving apparatus. In FIG. 5, a mark of three slashes indicates that a signal line is for three phases.
In the PWM power converter parallel running apparatus shown in FIG. 5, inverters INV1 and INV2 are connected in parallel, and connected with an electric motor M through interphase reactor(s) L_mut. In this case, a current control is largely divided into an output current control and a cross current control.
For the output current control, a deviation between a current command value or quantity Id_cmd, Iq_cmd and a current detection value or quantity Id_det, Iq_det is obtained, a PI control is performed by output current control sections 5a and 5b, and a dq inverse transformation is performed by a dq inverse transformer 2.
For the cross current control, a cross current compensating command value or quantity Vccc_cmp calculated by a cross current compensating section BalanceACR is superimposed or overlapped on a voltage command V_cmd outputted from the dq inverse transformer 2, and voltage command limitation is imposed by current command limiting sections 3a and 3b with output limit values of inverters INV1 and INV2. Voltage commands V1_cmd and V2_cmd are voltage commands after this voltage command limitation. On the basis of these after-limitation voltage commands V1_cmd and V2_cmd, PWM generating sections PWM1 and PWM2 deliver switching commands G1_H, G1_L, G2_H and G2_L to inverters INV1 and INV2, and thereby operate the motor M by driving the inverters INV1 and INV2 in this way.
An output current I_det of the overall system is measured by sensing inverter output currents I1 and I2 and adding these inverter output currents. Since the output current I_det is a three-phase current, a dq transformer 4 performs a dq transformation for performing the output current control, and delivers the current detection values Id_det and Iq_det. This dq transformation utilizes a phase Theta_det sensed by a sensor such as an encoder.